Some image forming apparatus such as a printer, a facsimile machine, a copier, a plotter, and a multifunctional machine may use a printhead comprised of a liquid spray head for spraying liquid droplets to a paper sheet to form an image while shifting the position of the sheet serving as a medium. The term “medium” may hereinafter be referred to as a “paper sheet”, but is not intended to limit the type of material used. Other terms such as “record medium”, “recording medium”, “transfer member”, “recording sheet” may as well be used to mean the same thing as the term “medium”. By the same token, terms such as “recording” and “printing” are used to mean the same as “image forming”.
In this disclosure, the term “image forming apparatus” refers to an apparatus for forming an image by spraying liquid on a medium made of material such as paper, thread, fiber, cloth, leather, metal, plastics, glass, wood, and ceramic. Further, the term “image forming” not only means attaching to a medium an image having semantic meanings such as characters and graphics, but also means attaching to a medium an image having no semantic meanings such as random patterns. Moreover, the term “ink” is not limited to ink in the narrow sense of the word, but refers to any liquid such as ink, resist, and DNA samples that can be used to “form image” in the sense of the term as described above.
In such image forming apparatuses (which may hereinafter be referred to as “inkjet printing apparatuses”), a sub-tank (also referred to as a buffer tank) for supplying ink to a printhead may be mounted on a carriage, while a main ink cartridge (also referred to as a main tank) may be mounted on an immovable part of the apparatus, such that the sub-tank is refilled with ink supplied from the ink cartridge. Alternatively, an ink cartridge that is an exchangeable liquid reservoir may be mounted together with a printhead.
In order to stabilize the spraying of ink droplets from a liquid spray head, it is vitally important to maintain the ink in the liquid spray head at a predetermined negative pressure (i.e., maintain the pressure applied to the ink inside the liquid spray head to a predetermined negative pressure). To this end, a negative pressure generating means is generally provided in the ink supply system that supplies ink to the liquid spray head. With this configuration, ink to which a negative pressure is applied by the negative pressure generating means is supplied to the liquid spray head.
The negative pressure generating means may generate a negative pressure by utilizing capillary suction by a spongy ink absorber contained in an ink cartridge, or may use an urging means such as a spring to outwardly urge an elastic material forming part of the ink cartridge to maintain a negative pressure inside the ink cartridge. Another type of negative pressure generating means has an ink cartridge positioned below the liquid spray head to apply a negative pressure to the ink by utilizing a water head difference.
In typical image forming apparatuses using a sub-tank, the supply passageway to supply ink from an ink cartridge to a sub-tank may be implemented as a flexible resin tube to increase latitude in installment and assembly inside the apparatus and maintainability. Further, an elastic film may be provided as a negative pressure generating means for the sub-tank. The use of an elastic film, however, gives rise to a problem in that the film may allow air passage through long-term use, resulting in air being introduced into the sub-tank. Moreover, air present inside the main tank, air introduced into the supply passageway at the time of installment of the main tank, and air dissolved in the ink may find their way into the sub-tank through the supply passageway to be accumulated in the sub-tank.
Patent Document 1 discloses providing a vent-to-atmosphere valve to switch the internal space of the sub-tank between a closed state and an open state and providing a vent passageway separately from an ink supply passageway to vent air inside the sub-tank, thereby preventing air accumulation in the sub-tank. This vent-to-atmosphere valve includes a valve seat made of an elastic material situated inside a vent-to-atmosphere cap, an iron ball serving as a valve plug, and a spring for urging the iron ball against the valve seat. The spring constantly urges the iron ball to maintain a closed state, thereby preventing the sub-tank from communicating to the open atmosphere, and preventing air from being introduced into the sub-tank. Further, a vent-to-atmosphere pin may be inserted into a holder to move the iron ball against the resisting urge applied by the spring to achieve an open state, thereby making the sub-tank communicate with the open atmosphere. In this state, ink is supplied to the internal space of the sub-tank to replace the air accumulated inside the sub-tank, ejecting the air from the sub-tank.
Patent Document 2 discloses providing an atmosphere introducing meander passageway and a valve along the atmosphere introducing passageway for the purpose of adjusting the pressure inside the tank.
Patent Document 3 discloses providing a filter that prevents dust and dirt from entering a tank when the tank communicates with the atmosphere.
The provision of a vent-to-atmosphere mechanism (valve) as described in Patent Document 1 makes it possible to vent air from a sub-tank to the atmosphere when ink is supplied from the main tank to the sub-tank. Further, the sub-tank is properly sealed for the purpose of maintaining a negative pressure after the ink is supplied to the sub-tank.
When the air-to-atmosphere valve of the sub-tank is opened at the time of initially supplying ink to the liquid spray head for the first time after shipment or at the time of recreating a negative pressure upon performing a maintenance work for recovering the nozzle function by a user, air in the atmosphere is sucked into the sub-tank upon communicating to the atmosphere because the interior space of the sub-tank is substantially in a hermetically sealed state. Due to the suction of air, dust and foreign material present in the atmosphere may enter the sub-tank through the vent-to-atmosphere valve.
If foreign material (including dust) enters the sub-tank to be mixed into the ink inside the tank, the printhead for forming image may suffer trouble such as a spray failure. Also, the contamination of the valve seat and valve plug constituting the vent-to-atmosphere valve by dust and foreign material causes deterioration in the airtightness of the valve, resulting in difficulties in recovering a hermetically-sealed state in the sub-tank. This may make it impossible to generate a negative pressure inside the sub-tank. As a result, ink may drip from the nozzles of the printhead to smear a print sheet and/or make it impossible to properly form images. When this happens, a recovery operation to discharge ink from the nozzles of the liquid spray head by use of a suction pump is necessary in order to reinstate a negative pressure. This is problematic because ink is wasted by the sucking operation to reinstate a negative pressure.
It should be noted that Patent Document 1 discloses applying grease to improve the airtightness of the valve. However, this may further deteriorate the airtightness since dust and foreign material in the atmosphere getting attached to the grease upon opening the valve may permanently remain.
In the configuration disclosed in Patent Document 2, the atmosphere introducing passageway is serpentine. When ink is supplied to the sub-tank, therefore, the pressure inside the sub-tank does not promptly reach the pressure of the atmosphere due to resistance along the serpentine passage way. This gives rise to a problem in that the ink supply time becomes lengthy.
Patent Document 3 discloses providing a filter for a valve. Since the filter is not provided on the same side of the valve as the atmosphere, it is not possible to remove dust and foreign material attached to the valve plug and valve seat.
Accordingly, there is a need to improve the airtightness of a liquid reservoir provided with a vent-to-atmosphere mechanism.
[Patent Document 1] Japanese Patent Application Publication No. 2005-169674
[Patent Document 2] Japanese Patent No. 3772859
[Patent Document 3] Japanese Patent Application Publication No. 2006-272900